Socketmap postfix protocol so unix sockets can be used etc.

Hi,

This adds the socketmap functionality so we can use unix sockets with postfix - which adds to security on shared hosting servers etc. It also implements those user/group changes from libcommon.

I have also done my best to update the documentation on the new options, and add the SRS check bits to postlicyd I'm currently using.

The RedHat init scripts I'm currently using I've also added, and quoted the source of them, and my modifications.

I've been running it for a couple weeks so it would also be good to hear if anyone else tries it and if it works OK :-)

Thanks,

Jason